A major challenge with production of natural resources such as oil, gas and water from wells is that the productivity gradually decreases over time. While a decrease is expected to naturally accompanies the depletion of the reserves in the reservoir, often well before any significant depletion of the reserves, production diminishes as a result of factors that affect the geologic formation in the zone immediately surrounding the well and in the well's configuration itself. For example, Crude Oil production can decrease as a result of the reduction in permeability of the rock formation surrounding the well, a decrease of the fluidity of oil or the deposit of solids in the perforations leading to the collection zone of the well.
In production wells, perforations aid the fluid from the formation seeping through cracks or fissures in the formation to flow toward a collection compartment in the well. Hence, the pore size of the perforations connecting the well to the formation determines the flow rate of the fluid from the formation toward the well. Along with the flow of oil, gas or water, very small solid particles from the formation, called “fines,” flow and often settle around and within the well, thus, reducing the pore size.
Solids such as clays, colloids, salts, paraffin etc. accumulate in perforation zones of the well. These solids reduce the absolute permeability, or interconnection between pores. Mineral particles may be deposited, inorganic scales may precipitate, paraffins, asphalt or bitumen may settle, clay may become hydrated, and solids from mud and brine from injections may invade the perforations. The latter problems lead to a flow restriction in the zone surrounding the perforations.
As a result of the reduction of productivity, of oil wells for example, the exploitation may become prohibitively expensive forcing abandonment of the wells.
Production wells of oil and gas, for instance, are periodically stimulated by applying three general types of treatment: mechanical, chemical, and other conventional techniques which include intensive rinsing, fracturing and acid treatment.
Chemical acid treatment consists of injecting in the production zone mixtures of acids, such as hydrochloric acid and hydrofluoric acid (HCI and HF). Acid is used for dissolving reactive components (e.g., carbonates, clay minerals, and in a smaller quantity, silicates) in the rock, thus increasing permeability. Additives, such as reaction retarding agents and solvents, are frequently added to the mixtures to improve acid performance in the acidizing operation.
While acid treatment is a common treatment to stimulate oil and gas wells, this treatment has multiple drawbacks. Among the drawbacks of acid treatment are: 1) the cost of acids and the cost of disposing of production wastes are high; 2) acids are often incompatible with crude oil, and may produce viscous oily residues inside the well; precipitates formed once the acid is consumed can often be more obnoxious than dissolved minerals; and 3) the penetration depth of active or live acid is generally low (less than 5 inches or 12.7 cm).
Hydraulic fracturing is a mechanical treatment usually used for stimulating oil and gas wells. In this process, high hydraulic pressures are used to produce vertical fractures in the formation. Fractures can be filled with polymer plugs, or treated with acid (in rocks, carbonates, and soft rocks), to form permeability channels inside the wellbore region; these channels allow oil and gas to flow. However, the cost of hydraulic fracturing is extremely high (as much as 5 to 10 times higher than acid treatment costs). In some cases, fracture may extend inside areas where water is present, thus increasing the quantity of water produced (a significant drawback for oil extraction). Hydraulic fracture treatments extend several hundred meters from the well, and are used more frequently when rocks are of low permeability. The possibility of forming successful polymer plugs in all fractures is usually limited, and problems such as plugging of fractures and grinding of the plug may severely deteriorate productivity of hydraulic fractures.
Another method for improving oil production in wells involves injecting steam or water. One of the most common problems in depleted oil wells is precipitation of paraffin and asphaltenes or bitumen inside and around the well. Steam or water has been injected in these wells to melt and dissolve paraffin into the oil or petroleum, and then all the mixture flows to the surface. Frequently, organic solvents are used (such as xylene) to remove asphaltenes or bitumen whose melting point is high, and which are insoluble in alkanes. Steam and solvents are very costly (solvents more so than steam), particularly when marginal wells are treated, producing less than 10 oil barrels per day. The main limitation for use of steam and solvents is the absence of mechanical mixing, which is required for dissolving or maintaining paraffin, asphaltenes or bitumen in suspension.
Empirical evidence have shown that seismic type waves may have an important effect on oil reservoirs. For example, following seismic waves, either from earthquakes or artificial induction, there is a rise in the fluid levels (water or oil), yielding an increase in oil production. A report on these phenomena is published by I. A. Beresnev and P. A. Johnson (GEOPHYSICS, VOL. 59, NO. 6, JUNE 1994; P. 1000-1017), which is included in its entirety herewith by reference.
Several methods using sound waves to stimulate oil wells have been described. Challacombe (U.S. Pat. No. 3,721,297) describes a tool for cleaning wells using pressure pulses: a series of explosive and gas generator modules are interconnected in a chain, in such a manner that ignition of one of the explosives triggers the next one and a progression or sequence of explosions is produced. These explosions generate shock waves that clean the well. There are obvious disadvantages of this method, such as potential damages that can be caused to high-pressure oil and gas wells. Use of this method is not feasible because for additional dangers including fire and lack of control during treatment period.
Sawyer (U.S. Pat. No. 3,648,769) describes a hydraulically controlled diaphragm that produces “sinusoidal vibrations in the low acoustic range”. Generated waves are of low intensity, and are not directed or focused to face the formation (rock). As a consequence, the major part of energy is propagated along the perforations.
Ultrasound techniques have been developed to increase production of crude oil from wells. However, there is a great amount of effects associated with exposing solids and fluids to an ultrasound field of certain frequencies and energy. In the case of fluids in particular, cavitation bubbles can be generated. These are bubbles of gas dissolved in liquid, or bubbles of the gaseous state of the same liquid (change of phase). Other associated phenomena are liquid degassing and cleaning of solid surfaces.
Maki Jr. et al. (U.S. Pat. No. 5,595,243) propose an acoustic device in which a piezoceramic transducer is set as radiator. The device presents difficulties in its manufacturing and use, because an asynchronous operation is required of a high number of piezoceramic radiators.
Vladimir Abramov et al., in “Device for Transferring Ultrasonic Energy to a Liquid or Pasty Medium” (U.S. Pat. No. 5,994,818) and in “Device for Transmitting Ultrasonic Energy to a Liquid or Pasty Medium” (U.S. Pat. No. 6,429,575), propose an apparatus consisting of an alternating current generator operating within the range of 1 to 100 kHz to transmit ultrasonic energy, and a piezoceramic or magnetostrictive transducer emitting ultrasound waves, which are transformed by a tubular resonator or wave guide system (or sonotrode) in transverse oscillations that contact the irradiated liquid or pasty medium. However, these patents are conceived to be used in containers of very large dimensions, at least as compared with the size and geometry of perforations present in wells. This shows limitations from a dimensional point of view, and also for transmission mode if it is desired to enhance production capacities of oil wells.
Julie C. Slaughter et al., in “Ultrasound Radiator of Dowhole Type and Method for Using It” (In U.S. Pat. No. 6,230,788), propose a device that uses ultrasonic transducers manufactured of Terfenol-D alloy and placed at the well bottom, and fed by an ultrasonic generator located at the surface. Location of transducers, axially to the device, allows the emission along a transverse direction. This invention proposes a viscosity reduction of hydrocarbons contained in the well through emulsification, when reacting with an alkaline solution injected to the well. This device considers a forced shallow circulation of fluid as a refrigeration system, to warrant continuity of irradiation.
Dennos C. Wegener et al., in “Heavy Oil Viscosity Reduction and Production,” (U.S. Pat. No. 6,279,653), describe a method and a device for producing heavy oil (API specific gravity less than 20) applying ultrasound generated by a transducer made of Terfenol alloy, attached to a conventional extraction pump, and powered by a generator installed at the surface. In this invention the presence of an alkaline solution is also considered, similar to an aqueous sodium hydroxide (NaOH) solution, to generate an emulsion with crude oil of lower density and viscosity, thereby facilitating recovery of the crude by impulsion with a pump. Here, a transducer is installed in an axial position to produce longitudinal ultrasound emissions. The transducer is connected to an adjacent rod that operates as a wave guide or sonotrode.
Robert J. Meyer et al., in “Method for improving Oil Recovery Using an Ultrasonic Technique” (U.S. Pat. No. 6,405,796), propose a method to recover oil using an ultrasound technique. The proposed method consists of disintegrating agglomerates by means of an ultrasonic irradiation technique, and the operation is proposed within a certain frequency range, for the purpose of handling fluids and solids in different conditions. Main oil recovery mechanism is based in the relative momentum of these components within the device.
The latter mentioned prior art generates ultrasonic waves via a transducer that is externally supplied by an electric generator connected to the transducer through a transmission cable. The transmission cable is generally longer than 2 km, which has the disadvantage of signal transmission loss. Since high-frequency electric current transmission to such depths is reduced to 10% of its initial value, the generated signal must have a high intensity (or energy), enough for an adequate operation of the transducers within the well. Furthermore, since the transducers need to operate at a high-power regime, water or air cooling system is required, which in turn poses great difficulties when placed inside the well. The latter implies that ultrasound intensity must not exceed 0.5-0.6 W/cm2. This level is insufficient for the desired purposes, because threshold of acoustic effects in oil and rocks is from 0.8 to 1 W/cm2.
Andrey a. Pechkov, in “Method for Acoustic Stimulation of Wellbore Bottom Zone for Production Formation” (RU Patent No. 2 026 969), disclose methods and devices for stimulating production of fluids within a producing well. These devices incorporate, as an innovating element, an electric generator attached to the transducer, and both of them integrated in the well bottom. These transducers operate in a non-continuous mode, and can operate without needing an external cooling system. The impossibility of operating in a continuous mode to prevent overheating is one of the main drawbacks of this implementation since the availability of the device is reduced. Moreover, because the generator is located in the wellbottom, and especially because of the use of high power, the failure rate of the equipment is likely to be high, thus raising the cost of maintenance.
Oleg Abramov et al., in “Acoustic Method for Recovery of Wells, and Apparatus for its Implementation” (U.S. Pat. No. 7,063,144), disclosure an electro-acoustic method for stimulation of production within an oil well. The method consists of stimulating, by powerful ultrasound waves, the well extraction zone, causing an increase of mass transfer through its walls. This ultrasonic field produces large tension and pressure waves in the formation, thus facilitating the passage of liquids through well recovery orifices. It also prevents accumulation of “fines” on these holes, thereby increasing the life of the well and its extraction capacity.
Kostyuchenko in “Method and apparatus for generating seismic waves” (U.S. Pat. No. 6,776,256) generates seismic waves in an oil reservoir for well stimulation by chemical detonation. A packer is lowered into the well, where a fuel and air mixture is injected, and then detonated, generating seismic waves that reach the well walls. Some problems may appear considering possible unwanted explosions and difficulties regarding the transportation of a fuel and air mixture deep into the well.
Kostrov in “Method and apparatus for seismic stimulation of fluid bearing formations” (U.S. Pat. No. 6,899,175) describe another device for seismic waves generation. Shock waves are generated when compressed liquid is discharged to the well casing, forming seismic waves in the well borehole. This device has a limited range of applications as it may be only used in injection wells.
Ellingsen in “Sound source for stimulation of oil reservoirs” (US patent application publication 2009/0008082) a seismic wave generator is presented. Pressurized gas from a compressor located on the surface is transported into the wellbore where it operates a sound source that emits the seismic waves. The main limitation of this device is that it cannot operate over 1 kHz.
Murray in “Electric pressure actuating tool and method” (U.S. Pat. No. 7,367,405) describes using a tool to stimulate a down-hole using mechanical waves. This tool comprises a housing having a chamber filled with liquid, where an electrical discharge is produced. The discharge vaporizes the liquid creating a shock wave that pushes a piston, thus generating a pressure wave in the surrounding fluid. However, the presence of moving parts in the down hole may present difficulties, for instance, to provide required maintenance.
In “The application of high-power sound waves for wellbore cleaning”, Champion et al., analyze techniques related to high power sound waves used in well stimulation, and indicate that a variety of techniques exists for the generation of sound waves, with one of the most common laboratory methods comprising the use of either piezoelectric or magnetostrictive type transducers. The piezoelectric devices employ a crystal that oscillates in response to an applied oscillating voltage, while the magnetostrictive devices employ an alloy that changes shape in the presence of a magnetic field and, creates a powerful force. In both cases, this study indicates that, the oscillatory movement generated is used to drive an acoustic transmitter element. The average power level of these devices is in the region of 0.5 watts/cm2, and the potential to increase this significantly is limited because of the presence of gas bubbles released by the periodic pressure oscillations within the fluid. Instead of this method based on transducers Champion et al. proposes the generation of high power sound waves by initiating a high voltage electrical discharge in a liquid medium—the electrolyte. This concept of sound wave generation has been practiced previously in the development and application of marine and downhole seismic “sparker” sources.
A high-energy electrical discharge, which may be of the order or several hundred joules, is triggered at a spark gap submerged in an electrolyte. Typical electrical-breakdown times in water can be engineered to occur in the nanosecond time scale. A high current flows from the anode to cathode, which causes the electrolyte adjacent to the spark gap to vaporize and form a rapidly expanding plasma gas bubble. After the discharge stops, the bubble continues to expand until its diameter increases beyond the limit sustainable by surface tension, at which point it will rapidly collapse (cavitation mechanism), producing the shock wave that propagates through the fluid and is used for wellbore cleaning. Previous work in the field has demonstrated that the creation of this transient acoustic shock wave, in the form of a pressure step function, has the potential to generate high power ultrasound with an intensity of greater than 50 watt/cm2.
Sidney Fisher and Charles Fisher in “Recovery of hydrocarbons from partially exhausted oil wells by mechanical wave heating” (U.S. Pat. No. 4,049,053) describe heating underground viscous hydrocarbon deposits, such as the viscous residues in conventional oil wells, by mechanical wave energy to fluidize the hydrocarbons thereby to facilitate extraction thereof. The latter invention comprises a system for generating mechanical waves located on the ground surface transmitting the waves to the bottom of the well.
Therefore, what is needed is a method and system for improving well productivity that do not present, or at least that minimize, the above-mentioned drawbacks of each respective prior art.